An automobile steering system has been developed with power-assisting effect obtained by means of an actuator, such as an electric motor. When the electric motor is used in assisting or aiding the driver's steering wheel turning effort for manipulating the front wheels, it is desirable, for energy saving, to actuate the electric motor only when a reaction imparted to the steering wheel from the front wheels of a car in motion is too great. A motor-assisted steering system has a torque sensor for detecting the steering torque transmitted from a steering wheel so as to actuate the electric motor, and impart assisting torque to the steering shaft.
One such power-assisted steering system is known from, for example, Japanese Unexamined Patent Publication No. 61-287,861 entitled "Power-Assisted Steering System For Automotive Vehicle" published Dec. 18, 1986. The power-assisted steering system, which is an in-line, self-contained power-assisted steering system, utilizes a frameless electric motor incorporated in a steering shaft structure or assembly. Because of an integral structure of the electric motor integrally contained in the steering shaft structure or assembly, the steering shaft structure must be disassembled every time a power-assisting unit is maintained or replaced.
Another power-assisting unit described in, for example, Japanese Unexamined Utility Model Publication No. 59-79467 entitled "Motor-Assisted Steering System" laid open May 29, 1984, comprises a drive steering shaft element secured to a steering wheel and a driven steering shaft element secured to a steering gear unit which are coupled together by means of a torsion spring member so as to allow a relative rotational motion therebetween, and clutch means disposed between a drive gear member driven by means of an electric motor and either one of the drive and driven steering shaft elements so that the motor output transmitted from the drive gear member is connected to or cut off from the one steering shaft element. The clutch means consists of cam surfaces formed in either one of the drive gear member and the one steering shaft element, and cam follower members interposed between the cam surfaces and the other of the drive gear member and the one steering shaft element and cooperated with the other steering shaft element.
In the motor-assisted steering system, when no relative rotational motion occurs between the drive and driven steering shaft elements, the cam follower member is held between the drive gear member and the one steering shaft element with a slight clearance, whereby the motor output transmitted from the drive gear member is cut off from the one steering shaft element. On the other hand, when a steering torque to be imparted to the steering wheel in order to manipulate the front wheels on turn is too great so that it will cause a relative rotational motion between the drive and driven steering shaft elements, the cam follower member is tightly grasped between the cam surface and the one steering shaft element, whereby the motor output transmitted from the drive gear member is connected to the one steering shaft element, so as to assist the driver in turning or manipulating the front wheels of a car in motion.
A problem in association with such motor-assisted steering systems is that, because each cam follower member acts on a cam surface upon right and left turns of the front wheels of a car in motion and the size and location of the cam follower member is determined depending on the dimensions of the cam surface and the one steering shaft element, the strut angle or the angle of contact of the cam follower member (which is defined by the angle between the cam surface and a tangent to a contact point of the cam follower member to the one steering shaft element) is substantially same for both right and left turns. If the strut angle is not suitably designed for right and left turns, the cam follower member will tend to slip when locking the clutch means or not to become loose when releasing the clutch means.